The most recently discovered mineral is krotite, which was recently discovered in a meteorite fragment. Krotite has never been seen in nature before now, and yet its origins reach right back to the birth of the solar system.
The mineral has been artificially created before now as a part of certain forms of high-temperature concrete, but mineralogists had had no idea that this building material had a natural counterpart. This natural krotite was discovered in a type of meteorite known as a chondrite, which means it's a particularly ancient space rock that dates back to the time of planetary formation 4.5 billion years ago.
Krotite itself is a compound of calcium, aluminum, and oxygen, and it can only form in temperatures of well over 2,500 degrees Fahrenheit. What's more, krotite doesn't require high pressures to form, which supports the idea that, even by the standards of ancient minerals, it's really, really old, dating right back to when the solar nebula began to condense, kicking off the formation of our solar system. I've heard that everything old is new again, but I've got to say - 4.5 billion years is pushing it.